1. Field of the Invention
This invention relates to semiconductor storage devices, and more particularly to an improved semiconductor storage device using alternating layers of different semiconductor materials to provide spatially separate potential wells for confinement of holes and electrons, where the state of the device depends upon whether or not electrons and holes are confined in these wells.
2. Description of the Prior Art
Many types of semiconductor memories are known in the art, including those using bipolar type transistors or field effect transistors in shift register circuits. Typically, these are active devices where the state of the device or circuit is continually refreshed during operation since it is basically a volatile type of storage.
Various photoconductor memories are also known in the art which generally involve deep traps for retention of electrons. These photoconductors are light activated and often shown degradation properties which are adverse to their successful use.
In contrast with these prior art semiconductor memories and photoconductors, the present invention seeks to provide controllable semiconductor memory devices in which electrical activation, or light activation, can be used for writing in the state of the device. Conduction band edge discontinuities and valence band edge discontinuities are used to provide confinement wells for holes and electrons, in contrast with prior art photoconductors where deep traps, due to impurities, etc., are used for retention of charge carriers.
Accordingly, it is a primary object of the present invention to provide a semiconductor memory device in which storage states can be maintained for long lifetimes.
It is another object of the present invention to provide semiconductor storage devices which are more efficient than prior semiconductor memory devices or photoconductors.
It is another object of the present invention to provide a semiconductor storage device in which the storage states of the device exist at zero bias, i.e., without retention voltages or power.
It is another object of the present invention to provide a semiconductor photoconductive material which does not require deep traps for retention of charge carriers.
It it another object of the present invention to provide a semiconductor photoconductor which does not require the use of impurity dopants for provision of charge trapping centers.
It is still another object of the present invention to provide a semiconductor storage device which is responsive to electrical or light activation having controlled valence band and conduction band edge discontinuities.
It is yet another object of the present invention to provide a semiconductor storage device in which charge carriers can be controllably retained for long periods of time.
It is a further object of the present invention to provide an improved semiconductor storage device having long lifetime, which can be electrically addressed for write, read, and erase operations.
It is a further object of the present invention to provide arrays of semiconductor storage devices providing three-dimensional confinement of electrons and holes in the devices.